STRATOSE THALLUS 79 



periderm, such as the ash, the Hchen emerges much earher and becomes 

 superficial. 



Black (or occasionally white) lines intersect the thallus and mark, as in 

 saxicolous lichens (Fig. 41), the boundary lines between different indi- 

 viduals or different species. The pioneer hyphae of certain lichens very 

 frequently become dark-coloured, and Bitter^ has suggested as the reason 

 for this that in damp weather the hypothallic growth is exceptionally 

 vigorous. When dry weather supervenes, with high winds or strong sun- 

 shine, the outlying hyphae^ unprotected by the thallus, become dark- 

 coloured. On the return of more normal conditions the blackened tips are 

 thrown off. Bitter further states that species of Graphideae do not form a 

 permanent black limiting line when they grow in an isolated position: it is 

 only when their advance is checked by some other thallus that the dark per- 

 sistent edge appears, a characteristic also to be seen in the crust of other 

 lichens. The dark boundary is always more marked in sunny exposed 

 situations: in the shade, the line is reduced to a mere thread. 



Bitter's restriction of black boundary lines to cases of encountering 

 thalli only, would exclude the comparison one is tempted to make between 

 the advancing hyphae of lichens and those of many woody fungi where the 

 extreme edge of the white invaded woody tissue is marked by a dark line. 

 In the latter case however it is the cells of the host that are stained black 

 by the fungus pigment. 



2. SQUAMULOSE LICHENS 



A. Development of the Squamule 



The crustaceous thallus is more or less firmly adherent to, or confused 

 with, the substratum. Further advance to a new type of thallus is made 

 when certain hyphal cells of soredium or granule take the lead in an 

 ascending direction both upwards and outwards. As growth becomes 

 definitely apical or one-sided, the structure rises free from the substratum, 

 and small lobules or leaflet-like squamules are formed. Each squamule 

 in this type of thallus is distinct in origin and not merely the branch of 

 a larger whole. 



In a few lichens the advance from the crustaceous to the squamulose 

 structure is very slight. The granules seem but to have been flattened out 

 at one side, and raised into minute rounded projections such as those that 

 compose the thallus of Lecanora badia generally described as "subsquamu- 

 lose." The squamulose formation is more pronounced in Lecidea ostreata, 

 and in some species oi P armaria; and the whole thallus may finally consist 

 of small separate lobes as in Lecidea lurida, Lecanora crassa, L. saxicola, 



1 Bitter i8qq. 



